cat shows full file content. less pages through large files — press Space to scroll and q to quit. head -n 20 shows the first 20 lines and tail -n 20 the last 20. tail -f follows a log file in real time.
Viewing File Content
cat file.txt # print entire file
less file.txt # page through (q to quit)
head -20 file.txt # first 20 lines
tail -20 file.txt # last 20 lines
tail -f app.log # follow file in real time
wc -l file.txt # count lines